Cellular phones have come into rapidly expanding use in recent years, resulting in a dramatic increase in the quantity of data communicated. For this reason, high capacity communication systems, such as the wideband code division multiple access (W-CDMA) system, are increasingly introduced for use.
A radio frequency power amplifier for use in the transmitter unit of a cellular phone terminal consumes more power than most other constituent elements of the terminal, and enhancing its efficiency is an essential requirement for extending the battery life. Further for use in a high capacity communication system, such as W-CDMA, reducing its distortion is indispensable along with enhancing the efficiency.
However, a class A or B amplifier normally used in a radio frequency power amplifier cannot achieve a high efficiency and a low distortion at the same time. A class A or B amplifier, when its output is low, is susceptible to little distortion but inefficient because of its linear action and, when its output is high, is efficient but susceptible to much distortion because it operates near or in saturation.
As a power amplifier that can achieve high frequency and low distortion at the same time, for instance an envelope elimination and restoration (EER) type amplifier is disclosed in U.S. Pat. No. 6,084,468, and is thereby made known to the public. One example of EER type amplifier described in the reference is shown in FIG. 11.
A radio frequency signal inputted into an input terminal 107 is distributed by a power divider 101 to an envelope detector 102 and a time delay element 104. The envelope detector 102 detects the envelope of the radio frequency signal. This envelope is amplified by an envelope amplifier 103, and supplied to the power supply terminal of a carrier amplifier 106. The time delay element 104 compensates for time delays in the envelope detector 102 and the envelope amplifier 103. The output signal of the time delay element 104 is subjected to amplitude limitation by a limiter 105 and amplified by the carrier amplifier 106.
The envelope of the radio frequency signal is once lost in the limiter 105, but as the envelope is supplied to the power supply terminal of the carrier amplifier 106, the lost envelope is restored by the carrier amplifier 106. Since the EER type amplifier is so designed that the carrier amplifier 106 always operates near or in saturation without depending on input power, it is highly efficient irrespective of whether the output is low or high. The problem of distortion, which is a disadvantage of operation near or in saturation, is solved by this recovery of the envelope of the radio frequency signal by the carrier amplifier 106, and a relatively distortion-free high power radio frequency signal is obtained at an output terminal 108.
In order to make the EER type amplifier highly efficient and relatively free from distortion, the efficiency should be enhanced and the distortion reduced not only of the carrier amplifier 106 but also of the envelope amplifier 103. An envelope signal, though lower in frequency of the carrier of radio frequency signals, in a wide band system such as W-CDMA, even the envelope has a wide frequency band, extending over a few MHz.
On the other hand, a DC—DC converter frequently used in the envelope amplifier 103 has a frequency band of at most 100 kHz or so, and accordingly the DC—DC converter cannot be directly used in the envelope amplifier 103 of a wide band system such as W-CDMA.
An example of envelope amplifier 103 disclosed in U.S. Pat. No. 6,084,468 is shown in FIG. 12. In this example of the prior art, parallel connection of a class S modulator 21 and a class B amplifier 3 is used. An envelope signal inputted to a terminal 5 is separated by a low pass filter 1 and a high pass filter 31 into low frequency components and high frequency components. The low frequency components are amplified by the class S modulator 21, while the high frequency components are amplified by the class B amplifier 3. The low frequency components and the high frequency components are synthesized after having passed a low pass filter 22 and a high pass filter 4, respectively, and a high power envelope signal is outputted to an output terminal 6.
The class S modulator 21 and the low pass filter 22, constituting a DC—DC converter, are highly efficient but narrow in bandwidth. On the other hand, the class B amplifier 3 is large in bandwidth but poor in efficiency. Therefore, by connecting the class S modulator 21 and the class B amplifier 3 in parallel, the overall bandwidth and high efficiency are attained.
Further in this example of the prior art, in order to enhance the efficiency of the class B amplifier 3, a power supply circuit consisting of a peak detector 32 and a regulator 33 is added. In this power supply circuit, the peak detector 32 detects the peak level of high frequency components and, by controlling the regulator 33 with this peak level, the power supply voltage to the class B amplifier 3 is minimized to enhance the efficiency of the class B amplifier 3.